The present invention relates to protective relay circuits for use in AC power distribution systems and more particularly to directional overcurrent relays used for transmission line protection.
Directional overcurrent relays are used in AC power transmission line protection systems to determine whether a fault is in the direction of the protected zone or in the "reverse" or non-protected direction. Negative sequence directional overcurrent relays are particularly useful in directional comparison relay schemes because they are essentially unaffected by such things as zero sequence mutual coupling.
Prior art negative sequence overcurrent trip units were primarily responsive to the level of negative sequence current; but had a restraint proportional to the positive sequence current; for example, I.sub.2 -KI.sub.1. Although such units perform satisfactorily, there was a probability of false tripping because of the reliance upon the level of negative sequence current by the unit even with the addition of the restraint proportional to the positive sequence current. In addition, prior art units operated relatively quickly on remote external faults thereby increasing the probability that the unit would operate before the external fault was cleared. Furthermore, prior art overcurrent trip units require settings which must be increased by the maximum negative sequence shunt charging current (due to the shunt capacitance on the line), based on maximum negative sequence voltage, V.sub.2 on the line. This was required to prevent a misoperation with a tripping scheme and to assure coordination with the remote overcurrent blocking unit in a blocking scheme. However, due to a high resistance ground fault, V.sub.2 will be substantially less than its worse case maximum value. With this approach, a higher setting on I.sub.2 -KI.sub.1 due to the fact that the worse case must be considered, limits the sensitivity of the prior art units to ground fault resistance.
Accordingly, it is an object of the present invention to provide a directional overcurrent relay for protection of an AC power transmission line, the operation of which enhances the security of the protective relaying system.
It is another object of the present invention to provide a directional overcurrent unit utilizing improved operating quantities which cause the unit to operate for fewer external faults than prior art units.
It is a further object of the present invention to provide a directional overcurrent unit that will operate correctly should a second fault occur on the transmission line during the period that one pole is open.
It is yet another object of the present invention to provide a directional overcurrent unit for use for protection of an AC power transmission line which has comparatively longer operating times on remote external faults than do prior art units thereby increasing the probability that the external fault will be clear before the unit can operate, thus diminishing the possibility of a misoperation where a false channel operation might occur during the fault.
It is a still further object of the present invention to provide a directional overcurrent unit which has increased sensitivity to high resistance ground faults.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.